Newbie with PWM measurement questions.

Streak

Enlightened
Joined
Jan 31, 2002
Messages
711
Location
ex South Africa now SoCal
Hello from South Africa to all you fellow flashaholics. I have been reading the messages for many months and took the plunge to register today.
I have built the 555 and Mosfet circuit as described by OddOne http://www.geocities.com/siliconvalley/bay/4397/.
The scope shows that the PWM is working as does thie 1M ohm dimmer.

Now For The Questions!!

Being PWM can one still accurately measure current drawn by the LED as well as voltage across its input?
Driving the circuit off about 4.7v with a 10 ohm limiting resistor the LED glows really bright at about 16ma. Playing around with the resistor I can get the LED to draw about 40ma.
What is the rule of thumb about the relationship between the current limiting resistor and whether you set the circuit for current or voltage for the LED? The specs say about 25mA and 3.6v. I can get the LED to blind me by varying either of these values.
What would be the best way to set the max. (ie. pot. turned to max brightness)such that the LED runs at its max. level.

The above is for 1 LED. As I add LED's you can see the exisitng ones dim slightly as the other's drop the voltage. I am currently running 3 LED's off the circuit. How many could I safely run off this circuit without adding more Mosfets?

Next project will be to build a DC regulating circuit (same as step up/step down??).
I have seen you all talking about the circuits you have built using samples from Maxim or National. Few of these components are avilable here and those companies do not seem to have sample programs for South Africa. I do not mind oredring some components but want to make sure that the circuit I build is flexible with a low component count. I have never played with SMD devices but can always learn.
I have seen the basics from 2222 plus 20 turn ferrite bead to the very fancy IC based circuits.
Please recommend a circuit for doing this.

Thanks in advance. This BB has given me many hours of reading pleasure. If only we had access to all of the toys you guys have.
 

bikeNomad

Newly Enlightened
Joined
Sep 8, 2001
Messages
166
Location
Stanwood WA
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Streak:
Being PWM can one still accurately measure current drawn by the LED as well as voltage across its input?<HR></BLOCKQUOTE>
Sure. You measure voltage drop across the series resistor with your scope. I'd recommend a bypass cap across the supply voltage, and I'd ditch the 470K resistor on the MOSFET base. The 10K could be removed too, or if you are getting ringing on the base drive you could put in (say) a 100 ohm resistor.

If you have a 2-channel scope, you can see the current waveform by putting one probe on either end of the current limiting resistor and setting your scope to subtract the channels.
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
Driving the circuit off about 4.7v with a 10 ohm limiting resistor the LED glows really bright at about 16ma. Playing around with the resistor I can get the LED to draw about 40ma.<HR></BLOCKQUOTE>As measured how? If you are using a multimeter there, you're either measuring average or RMS current (depending on the meter). Either is not answering the important question of peak current (see your LED datasheet). I'd choose the current limiting resistor conservatively by taking supply voltage minus LED voltage and dividing by desired LED peak current. Then I'd use the PWM adjust to change the brightness. <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
What is the rule of thumb about the relationship between the current limiting resistor and whether you set the circuit for current or voltage for the LED? The specs say about 25mA and 3.6v. I can get the LED to blind me by varying either of these values.<HR></BLOCKQUOTE>
You can't change the voltage directly with this circuit; you can only set the peak current (with the resistor) and the duty cycle (with the 555). <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>
The above is for 1 LED. As I add LED's you can see the exisitng ones dim slightly as the other's drop the voltage. I am currently running 3 LED's off the circuit. How many could I safely run off this circuit without adding more Mosfets?
<HR></BLOCKQUOTE>
Depends on the FET and its heatsinking. And the power rating of the resistor(s) and its heatsinking.
 

Streak

Enlightened
Joined
Jan 31, 2002
Messages
711
Location
ex South Africa now SoCal
Thanks for a comprehensive reply.
I did not mention that I am using a variable voltage power supply to do the testing so I do have one more variable to play with.
I set the input voltage at say 9v and adjusted the series resisistor accordingly then at say 4.7v and adjusted again. I was trying to get a feel for the optimum operating voltage as the original circuit called for 9 to 18 volts which seemed very high.

Any thoughts on a regulating circuit?
 
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